In the internal combustion engine, because of its structure causing fuel to explode at the top dead center of a piston in the bore and pushing down the piston by the explosion, the upper side of each cylinder bore wall has a higher temperature and the lower side has a lower temperature. Therefore, a difference arises in thermal deformation amount between the upper and lower sides of the cylinder bore wall. The upper side expands largely while the expansion of the lower side is smaller.
As a result, the frictional resistance between the piston and the cylinder bore wall increases, and this becomes a factor degrading fuel consumption. Therefore, it is required to reduce the difference in thermal deformation amount between the upper and lower sides of the cylinder bore wall.
Therefore, to make wall temperatures of respective cylinder bore walls uniform, it has been attempted to control the cooling efficiency by the cooling water on the upper and lower sides of the cylinder bore wall by installing a spacer in a groove-like cooling water passage to adjust the flow of cooling water in the groove-like cooling water passage. For example, Patent Literature 1 discloses an internal combustion engine cooling heat-transfer medium passage partition member, which is a passage partition member for partitioning a groove-like cooling heat-transfer medium passage into a plurality of passages when disposed in the groove-like cooling heat-transfer medium passage formed in a cylinder block of an internal combustion engine, including a passage division member formed to have a height not reaching the depth of the groove-like cooling heat-transfer medium passage and serving as a wall portion for dividing the groove-like cooling heat-transfer medium passage into a bore-side passage and an anti-bore-side passage, and a flexible lip member formed from the passage division member in a direction toward an opening portion of the groove-like cooling heat-transfer medium passage, and having a tip edge portion formed of a flexible material so as to extend beyond one inner surface of the groove-like cooling heat-transfer medium passage, so that the tip edge portion comes into contact with the inner surface by its own deflection restoration force, at an intermediate position in the depth direction of the groove-like cooling heat-transfer medium passage after completion of the insertion into the groove-like cooling heat-transfer medium passage, for separation into the bore-side passage and the anti-bore-side passage.
However, according to internal combustion engine cooling heat-transfer medium passage partition member disclosed in Patent Literature 1, since wall temperatures of cylinder bore walls can be made uniform to some extent, the difference in the thermal deformation amount between the upper and lower sides of the cylinder bore wall can be reduced. However, in recent years, it is required to further reduce the difference in the thermal deformation amount between the upper and lower sides of the cylinder bore wall.
Therefore, in recent years, wall temperatures of cylinder bore walls are made uniform by using a heat retention tool for active heat retention of a cylinder bore-side wall surface in a middle lower region of the groove-like cooling water passage of a cylinder block. Further, to attain effective heat retention of the cylinder bore-side wall surface in the middle lower region of the groove-like cooling water passage, the adhesion of the heat retention tool to the cylinder bore-side wall surface in the middle lower region of the groove-like cooling water passage is required to be high.